Dispersion

Figure 1: The spreading of light

Figure 1: The spreading of light

The change in the propagation constant for different waves is called dispersion.
Dispersion occurs because the spreading of a wave is dependent on
the wavelengths' interaction with the refractive index of the medium.
Different wavelengths travel at different speeds in the medium.
Material dispersion is a function of the source spectral width.

Basically, dispersion is the spreading of light as light propagates through a
more dense medium. The speed difference of waves of different frequencies shows
oneself particularly clearly in some media e.g. at a prism from glass: white
sunlight is taken to pieces into his spectral colors in this.

This phenomenon leads to the rise of the rainbow by water droplets in the atmosphere.